The Family That Nests Together, Rests Together

A Cape Gannet breeding colony at Bird Island Nature Reserve near Lamberts Bay, Western Cape, South Africa. Photo by Octagon, image from Wikipedia.

Fossils tell stories. I’ve written about this beautiful fact over and over again, but the multiple perspectives on prehistoric life afforded by even the most mundane petrified scrap never cease to impress me. Fossils are not merely individual facts of nature that educate us simply by their identification and accumulation. They are more than that – primeval touchstones that can send us down any number of pathways as we trace evolutionary history. Such ancient inspirations can be as simple as a fragment of turtle shell or a shed trilobite exoskeleton, and, as recently shown by paleontologist Gareth Dyke and colleagues, as grand as the remnants as a drowned bird colony.

The details of the incredibly rich deposit were published earlier this year in Naturwissenschaften. As described by Dyke and co-authors Mátyás Vremir, Gary Kaiser, and Darren Naish, the small boneyard is a short, seven-inch-thick lens of mudstone packed with eggshell and bones from a colony of archaic birds that nested together right at the end of the Age of Dinosaurs.

Vremir noted the existence of the shell-filled rock in a 2010 survey, but the new paper draws out what the site can tell us about the biology and mating habits of prehistoric birds called enantiornithines. These birds are part of a long-lost avian radiation – creatures between modern birds and their feathery non-avian dinosaur forebears. Enantiornithines could fly and resembled the typical bird archetype, but many retained the teeth and hand claws of their raptor-like ancestors, too.

Given the state of the evidence, it isn’t possible to tell which species of enantiornithine nested in Late Cretaceous Transylvania. The deposit is effectively a slurry of eggshell with a few bones and some matrix. The bones resemble those of a primordial bird called Enantiornis found in the Late Cretaceous of Argentina, but, lacking more complete skeletal material, we don’t yet know exactly what species the Transylvania specimens represent. Nevertheless, such a dense collection of egg remnants and bird bones indicates that behaviors we associate with modern birds actually reach far back into the fossil record, when avian and non-avian dinosaurs still lived alongside each other.

In this case, the story isn’t just in the fossils. The way the shells and bones were laid down provides critical clues. Geological context is essential for figuring out how particular creatures came to rest in the positions that paleontologists have found them in. According to Dyke and collaborators, the details of the site show that the shell-packed swath was created when a local flood transported eggs and birds into a shallow pond created by rising water. A handful of complete fossil eggs attest to the fact that the various elements of the bird colony had not been transported far from the nesting site.

The hypothesized proximity of the bird colony with the site where the eggs and bones were dropped might mean that the prehistoric birds had a penchant for aquatic prey. Modern birds that nest near water tend to be “aquatic foragers”, Dyke and co-authors point out, and the unfortunate enantiornithines may have followed the same pattern. Yet the fact that this behavior goes back over 66 million years is a puzzle. Modern shorebirds are strong flyers who can gather food from a wide area to bring back to their nests. While enantiornithines could fly, they weren’t quite so skilled in the air. Without more complete skeletal material, it’s hard to say whether the adults in the drowned bird colony would have been able to bring home meals from afar. Instead, Dyke and colleagues suggest, the Cretaceous birds might have timed their breeding season with a yearly abundance of local prey. Floods would sometimes overwhelm their nests, a tragic turn of circumstance that still happens to birds today, but at least one of these catastrophes locked a snapshot of these ancient birds in stone.

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